1. Field of the Invention
The present invention relates to a valve guide which is manufactured by a sintered alloy and used for an engine, and more particularly, to a valve guide which has high wear resistance and can prevent appearance of scuffing on a surface of a valve stem associated with the valve guide, and to a process for manufacturing the valve guide.
2. Description of the Prior Art
For use of engines of automobiles and the like, various valve guides manufactured by sintered alloy have been provided. Such type of the valve guide includes some hard phase in order to obtain wear resistance. For example, the applicant of the invention discloses a valve guide including hard phase consists of Fe--P--C eutectic compound (called steadite) in Japanese Patent Publication Nos. 1980-34858 and 1989-52463. The sintered alloy valve guide is manufactured in a way that a green compact is sintered, and is pressed fit into a cylinder head of an engine. Then, reaming is carried out to the inner surface of the sintered body, so that the valve guide is completed. Lubricating oil is provided to the valve guide by an oil feeding system provided to the engine, so that lubricating oil flows through pores in the valve guide, and exudes to a sliding portion between the valve guide and a valve stem supported thereby.
When quantity of lubricating oil exuding to the sliding portion is not enough, the surface of the valve guide becomes scratched and chipped by wear, that is to say, scuffing appears on the valve stem. On the contrary, when lubricating oil can easily flow through the pores, a large quantity of lubricating oil exudes to the sliding portion. As a result, lubricating oil leaks from the sliding portion and is sucked into the engine chamber due to negative pressure thereof, so that exhaust gas includes white smoke. Therefore, the valve guide is required to flow suitable quantity of lubricating oil through the pores. Furthermore, the valve guide is required to have not only wear resistance but also machinability which is important quality. However, as mentioned above, because the valve guide manufactured by sintered alloy includes hard phase dispersed in the structure, machining of the valve guide is difficult. Therefore, the applicant proposed a valve guide having improved machinability in Japanese Patent Laid Open No. 1992-57140 maintaining wear resistance of the valve guide disclosed in the above mentioned Japanese Patent Publication No. 1980-34858. However, a need exists for a valve guide having further improved machinability.
As mentioned above, the valve guide is required to have various performances at high level.